The invention lies in the semiconductor technology field and pertains, more specifically, to a method for producing a metal/metal contact as part of a multilayer metallization in an integrated circuit on a semiconductor wafer and to a device for carrying out preliminary processes as part of producing metal/metal contacts during a multilayer metallization in an integrated circuit on a semiconductor wafer.
During the wiring of circuit elements in an integrated circuit on a semiconductor wafer, techniques have been developed for wiring in a plurality of superposed levels, leading to a savings in chip surface area and to a reduction in the interconnect lengths. In the case of silicon-based integrated circuits, the metals aluminum and copper are primarily used for the metallization, since, on account of their low resistivity, they allow high current intensities. For the multilayer wiring of integrated circuits, the metal levels are arranged above one another, in each case separated from one another by insulation layers, and are connected to one another via contact openings. The latter are known as vias or via openings.
The necessary metal to metal contacts are thereby produced in a customary way using planar technology. For this purpose, an insulation layer, preferably an SiO2 layer, is produced on an interconnect level which has already been formed. Then, a light-sensitive resist is applied, which is then exposed via a mask bearing the pattern of a design level of the contact openings. Then, the exposed resist is removed and the SiO2 layer undergoes anisotropic etching with the aid of a plasma etch. Then, the remaining photoresist mask is removed over the entire surface.
The main component of the photoresists used is predominantly an organic polymer, which can preferably be removed using a plasma-etching step in an oxygen atmosphere. During the plasma etching of the SiO2 layer for the purpose of forming the contact hole and during the subsequent plasma etching of the photoresist mask, however, polymer residues usually accumulate in the etched contact hole and have to be removed prior to a metal deposition for the purpose of filling the contact hole. According to the customary method, the cleaning operation for removing polymer is carried out as a wet-chemical etch, in which the semiconductor wafer is immersed in a chemical bath. However, the wet film, which accumulates on the wafer surface during the wet-chemical etch leads to undesirable oxidation of the metal tracks in the region of the contact opening. Oxidation of this nature is also brought about by the fact that, when the semiconductor wafer is removed from the chemical bath, it generally comes into contact with air.
Therefore, prior to the metal deposition for the purpose of filling up the contact holes, it is necessary to use a further etching step, which is preferably carried out as a dry etch using an argon plasma, to remove the oxide on the metal track in the contact hole. However, this etching process leads to an undesirable roughening of the contact surface and, furthermore, to a high process temperature, which causes difficulties in particular for the deposition of aluminum in order to fill the contact opening.
The risk of oxidation of the metal surface in the contact hole also exists if a thin nitride intermediate layer has been introduced prior to the deposition of the SiO2 layer for insulation of the metal tracks, as is the case in particular for passivation of copper interconnects. This nitride layer on the bottom of the contact holes is, according to the customary procedure, removed after the removal of the polymer residues, preferably by a plasma-etching method. When the semiconductor wafer is transported out of the plasma-etching installation into the metal deposition installation, however, a film of moisture is also formed on the wafer surface, leading to oxidation of openings of the contact holes, so that in this case too oxide has to be removed prior to the deposition of metal.
U.S. Pat. Nos. 5,661,081; 6,025,255; 5,817,579 and 6,013,574 described methods for producing metal/metal contacts as part of a multilayer metallization. There, an insulation layer is applied to a metal level of the semiconductor wafer, then a photolithography step is carried out in order to define the contact holes, the contact holes are produced by anisotropic etching, in a following cleaning process the photoresist mask or organic impurities which form are removed, and finally the contact holes are filled as part of a metal deposition step. In this known method, a layer is formed between the cleaning process for removal of the photoresist mask and the other impurities and the metal deposition step on the bottom of the contact holes, and this layer has to be removed by additional etching prior to the metal deposition.
It is accordingly an object of the invention to provide a method and a device for forming metal on metal contacts, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and improves the process management for producing metal/metal contacts in integrated circuits on a semiconductor wafer, and in particular to make the process less expensive.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing a metal/metal contact as part of a multilayer metallization in an integrated circuit on a semiconductor wafer. The method comprises the following method steps:
applying an insulation layer to a metal plane;
carrying out a photolithography step using a photoresist mask to define contact holes on the insulation layer;
anisotropically etching the insulation layer to produce contact holes; and
without interrupting the process vacuum:
removing the photoresist mask by chemical dry etching; and
removing organic impurities accumulated during the dry etching step by chemical-physical dry etching; and
depositing metal to fill the contact holes.
In other words, according to the invention, the processes for the production of metal/metal contacts as part of a multilayer metallization in an integrated circuit on a semiconductor wafer are controlled in such a way that, after the application of an insulation layer to a metal level, a lithography step using a photoresist mask is carried out in order to define contact holes on the insulation layer, and then anisotropic etching of the insulation layer is carried out in order to produce the contact holes. Then, in a vacuum, a chemical dry etch, preferably a plasma etch in order to eliminate the photoresist mask, a chemical-physical dry etch, preferably a reactive ion etch in order to eliminate organic impurities which accumulate during the plasma etching, and a metal deposition in order to fill the contact holes are carried out in succession.
With this process management, in which the preliminary processes prior to the metal deposition for filling the contact holes are carried out without interruption to the vacuum, it is possible to eliminate the need for an additional process step in order to remove an oxide layer in the bottom of the contact holes, so that the speed of the process is significantly improved and, furthermore, a cost saving can be achieved. Furthermore, avoiding an additional etching process in order to remove oxide means that the wafer temperature can be kept at a low level prior to filling of the contact holes.
According to the invention, the plasma etch which is used to eliminate the photoresist layer and the reactive ion etch which is used to eliminate organic impurities which accumulate during the plasma etch are carried out in a common vacuum chamber, leading to a considerable cost saving which, furthermore, also saves space in the clean room. Furthermore, it is possible to dispense with additional wafer transport, which would involve the risk of oxidation.
In accordance with an added feature of the invention, the chemical dry etch to eliminate the photoresist mask is carried out with an etching gas excited to form a plasma through application of a high-frequency voltage or by the introduction of microwaves.
In accordance with an additional feature of the invention, the chemical-physical dry etching to eliminate the inorganic impurities is carried out by a reactive ion etching technique.
In accordance with a preferred embodiment of the invention, the etching gas used in the chemical dry etching step is oxygen.
According to one embodiment of the invention, the photoresist mask is isotropically etched away by a plasma-excited gas stream using a chemical dry-etching step, and then the organic impurities, which accumulate are removed using a chemical-physical dry-etching step with ion bombardment. These two etching techniques can be carried out particularly easily in a vacuum chamber using substantially the same equipment.
In accordance with again an additional feature of the invention, the metallization is a copper metallization and, the method further comprises, following the chemical-physical dry etching for eliminating the organic impurities, carrying out a further chemical-physical dry-etching step in the vacuum for removing a nitride layer at a bottom of the contact holes.
In this further preferred embodiment, where copper is used for the metallization, the nitride layer at the bottom of the contact holes is also etched away using a similar chemical-physical dry-etching step as that used to remove the organic impurities, all the etching processes, namely the removal of the photoresist mask, the etching of the organic impurities and the removal of the nitride layer, being carried out in a common vacuum chamber. In this way, equipment costs and the area required in the clean room are considerably reduced. Furthermore, it is possible to dispense with an additional wafer transport process.
With the above and other objects in view there is also provided, in accordance with the invention, a device for producing a metal/metal contact in a multilayer metallization of an integrated circuit on a semiconductor wafer, that is, a device for carrying out the above-summarized method. The device comprises:
a process chamber having a vessel formed with a process-gas inlet, a connection for a vacuum pump, electrodes connected for generating a high frequency and a wafer mount;
the vessel being configured for carrying out, in succession, in a vacuum, a chemical dry etch for removing a photoresist layer, serving as a mask for defining contact holes on an insulation layer arranged on a metal level on a semiconductor wafer, and a chemical-physical dry etch for removing organic impurities in the contact holes accumulating during plasma etching; and
the process chamber forming part of an installation for metal deposition, whereby the semiconductor wafer is enabled to be transported out of the vessel and into a metal-deposition reactor without interrupting the vacuum.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a novel process management for producing metal/metal contacts, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.